The present invention relates to improved fixed bed attrition resistant catalysts and to their use in exothermic chemical processes, such as vapor phase oxidation of olefinically unsaturated hydrocarbons to their corresponding unsaturated aldehydes, particularly propylene to acrolein.
Various oxidation catalysts have been proposed for use in vapor phase catalytic oxidation of olefinically unsaturated hydrocarbons to produce corresponding unsaturated aldehydes with a view towards enhancing catalyst life and selectivity for desired unsaturated hydrocarbon products without reducing rates of conversion of olefin raw material. Such known oxidation catalysts generally contain combinations of alkali metals, alkaline earth metals, V, Cr, Mo, W, Mn, Fe, Co, Ni, Cu, Zn, In, Tl, Sn, Sb, Bi, P, As among other elements.
U.S. Pat. No. 3,454,630 describes a process for converting propylene and isobutylene to the corresponding unsaturated aldehydes and carboxylic acids in the presence of a catalyst of the elements Ni, Co, Fe, Bi, P, Mo and oxygen.
U.S. Pat. No. 3,778,386 describes a vapor phase oxidation process in which propylene can be converted to acrolein utilzing a catalyst containing the following elements on a suitable carrier or binder: EQU Ni.sub.a Co.sub.b Fe.sub.c Bi.sub.d L.sub.e M.sub.h Mo.sub.f O.sub.g
wherein Ni, Co, Fe, Bi, Mo and O are the elements nickel, cobalt, iron, bismuth, molybdenum and oxygen, respectively; L is phosphorus, arsenic or boron and M is potassium, rubidium or cesium including mixtures; and wherein a and b are 0 to 15, while a+b is 2 to 15, c is 0.5 to 7, d is 0.1 to 4, e is 0 to 4, f is 12, g is 35 to 85 and h is 0.01 to 0.5. This combination of elements represents a family of highly preferred catalysts for the oxidation of propylene to acrolein.
Generally, catalysts used in the vapor phase oxidation of olefinically unsaturated hydrocarbons have been prepared conventionally as pills or tablets having an essentially uniform distribution of catalyst throughout. Catalysts prepared in the manner can be extremely active when used in fixed bed exothermic oxidation reactions. In some cases, however, "hot spots" can cause large amounts of undesirable by-products to be produced, due to the fact that the heat generated by the reaction cannot be dissipated efficiently by normal heat transfer techniques.
One method which has been tried to overcome this difficulty is to use inert catalyst supports which contain the catalytically active materials coated in or on the support. This reduces the amount of catalytically active material in the reactor and cuts down undesirable catalytic activity, thereby better controlling the heat produced in the reaction and substantially eliminating "hot spots". However, such coated catalysts often exhibit a significant degree of attritability, i.e., they break down in use to give significant amounts of catalyst fines. As these fine particles accumulate in the reactor, high pressure drops develop which must be overcome by applying higher pressures. Once the pressure reaches an unacceptably high level, the reaction must be stopped even though the catalyst may still retain good activity.
Attempts have been made in the past to provide active supported catalysts having acceptable attrition resistance. For example, U.S. Pat. No. 3,341,471 describes attrition resistant solid catalysts prepared by mixing finely divided catalytically active components with an aqueous silica sol, forming a paste or slurry, drying the mixture, heat-treating the finished finely divided catalyst and then pelletizing, yielding a catalyst having catalytically active material fairly uniformly distributed throughout the pellets. This catalyst, however, when used in a fixed bed reactor in reactions such as the oxidation of olefins to oxygenated hydrocarbons, still has a tendency to result in undesirable "hot spots".
U.S. Pat. No. 4,077,912 describes a procedure for producing a coated catalyst on an inert porous support. The catalyst is produced by wetting a porous inert support with an inorganic or organic liquid, water and petroleum ether being specifically disclosed, in such a manner that the resulting wetted support does not have the appearance of free liquid in contact with the support particles, and then gently agitating a catalytically active material onto the wetted support surface. The resulting catalyst is said to have good attrition resistance. The catalytically active material used in this catalyst is the same catalytically active material described in U.S. Pat. No. 3,778,386, earlier discussed. The resulting catalyst, although having good activity and selectivity for the conversion of propylene to acrolein, in many cases has an unacceptably high attrition rate.
In general, the catalysts described in the aforementioned patents have a lower attrition resistance than the catalysts of the present invention.